SOA Serial Number Wrong

I was just looking through a zone and noticed that the SOA serial number is 1 month behind. I just updated a zone yesterday 06/21/04 and the SOA is 20040521xx which is a SOA serial number for 05/21/04. How do I change this so it reflects the correct month?

I'm not sure what it is. This is the Serial number date I see when I go to DNS edit DNS zone within WHM for this particular zone. I can make an update, save it, and it immediately changes on DNS reports to a subsequent number but is still 1 month old as it shows in WHM.

Looking at another DNS zone, it's time stamp shows 2004032403. I made a change and it updated to 2004032404 not 2004062201 like I thought it would. Is this normal?

Doing 'date' in SSH shows the correct date so it looks like the server is on the correct date. I'm not sure if I should just update the serial numbers manually or leave them.

Here is my uninformed guess. The zone record is originally created on 2004032403. When the zone file is updated you get 2004032404 with the 04 being the revision number. Unless the zone file is re-created the date stamp will stick with it but the revision number changes..
Could be tested by creating a new zone or deleting a zone and re-creating it. If it were me and there weren't any problems with the zones I would probably leave it be.
Like I said just a guess.